Field of the Invention
The invention concerns a femoral component of a hip prosthesis intended to be implanted without cement, and comprising a shaft adapted to be driven into the medullary cavity of the femur after removal of the head by cutting the neck, said shaft tapering to a lower end from an upper end, from which projects a neck carrying an added spherical head adapted to form a ball-and-socket joint with a cotyloid component so as to reproduce the original natural joint.
(b) Description of the Related Art
It is well known that, in their structure, hip prostheses form a ball-and-socket joint, with a female component reconstituting the cotyloid cavity in the hip bone (ilium), and a male component carrying a spherical head fixed at the end of a neck, the neck being joined to a shaft anchored in the medullary cavity of the femur. The invention concerns this latter component, the femoral component, and refers to the cotyloid component only as far as concerns its complementarity with the femoral component.
It will be understood that as the prosthesis is intended to be substituted for the natural joint of the head of the femur in the cotyloid cavity of the hip bone in conditions as close as possible to the healthy natural joint, the elements of the prosthesis components which take the place of the bone elements of the joint will be designated by the same names as the bone elements, their orientations and dimensions will, unless otherwise indicated, be similar to those of the elements of the corresponding natural joint, and the reference to their spatial positions will be given to their implanted position.
More particularly it will be considered that, relative to the general axis of the femur, considered as approximately straight and vertical, the head is offset in a frontal plane on the internal side at the end of the neck, orientated in this front plane to form an angle of about 135.degree. with the axis of the femur, and that the axis of the neck connects with the axis of the femur through a curvature of the axis of the medullary canal in the region of the trochanter, this axis forming substantially the neutral axis of the femur considered as a load-bearing member.
There are two main types of femoral component for hip prostheses, the components which are sealed with a polymerised organic cement in the medullary cavity and the components which are jammed by force into this cavity, the growth of the spongy bone in contact with the surface of the shaft, suitably formed with numerous cavities and projections, providing an anchorage by interpenetration of the bone and the surface of the shaft.
The components which are sealed by means of an organic cement have a certain number of disadvantages; during the installation of the prosthesis, it is necessary to wait, after the sealing of the shaft, until the cement has acquired sufficient resistance by polymerisation before proceeding with the intervention. The setting of the cement by polymerisation releases some heat and it leads to a certain swelling of the cement which sometimes causes distressing pains. The presence of the cement is not favourable to the growth of the spongy bone, and it can happen that in the course of time the sealed shaft works loose, which requires a fresh intervention. Then, during a new installation of a prosthesis, the elimination of the previous cement can be difficult.
It has been recommended to use femoral components held by jamming in the medullary cavity. The shaft has a radius of curvature different from that of the axis of the medullary cavity at the connection with the neck of the femur. When this shaft has been driven into the medullary cavity, it bears on the walls of this cavity in three zones: at the two ends, the point of the shaft and the root of the neck it bears through its most concave internal generatrix, and in the region of the trochanter it bears through its dorsal generatrix, in an intermediate zone between the ends, the dorsal generatrix being opposed to the internal generatrix in the frontal plane. As already indicated, the exterior face of the shaft has a surface condition, sometimes called madreporic, with cavities and projections, to encourage the penetration and accretion of growing spongy bone. Other femoral components are massive and jam for all of their length. Their shaft is generally longer than that of those previously described.
Study of the behaviour of such femoral components after installation has shown that the locking thus obtained is not without disadvantages. The stress of the shaft creates pain for the patient. In other respects, it has become evident that while locking against a rotation of the shaft is indispensable, good functioning of the prosthesis involves a longitudinal suppleness, with a play of the order of a millimeter. Indeed if the metal, generally titanium for reasons of weight, biological compatibility, chemical inertia and suitability for machining, has a modulus of elasticity removed to the least extent from that of the cortical bone, it does not reproduce the suppleness and shock absorption of the natural bone in the region of the trochanter and the neck. Now, with jamming at three points, suppleness is obtained by longitudinal displacements of the shaft, which cause clearances at the bearing zones, but these clearances do not sufficiently ensure locking against rotation. Massive femoral components do not have the required suppleness.
It has been proposed also (EP-A-0 131 178) to provide a femoral component with a pin, carrying the head of the joint, which narrows from the top towards the bottom, and an assembly of wedge pieces which are freely guided in channels cut in the pin. The friction between wedge pieces and channels is much weaker than the friction which arises between the bone and the exterior surfaces of the wedge pieces, which are adapted to be adherent. Loading the pin brings about a spreading apart of the wedge pieces; if the medullary cavity enlarges, the driving-in of the pin avoids a working loose between the bone and the component. The document states that the wedge angle of the pin is smaller than the angle of friction between the pin and the wedge pieces in such a manner as to prevent a to-and-fro play between these parts.
Such an arrangement prevents perfectly any rotational play of the component relative to the bone, but at the cost of the suppression of all longitudinal suppleness. The disadvantages of the massive components are found again.